Release-time helpers for the standalone taskweft binary (issue #53).

wrap/1 is the final mix release step. It packages the assembled release into a self-contained per-triplet binary with Burrito, but only when a zig cross-compiler is available (Burrito's backend) or the build explicitly opts in via TASKWEFT_BURRITO=1. Otherwise it returns the release untouched, so mix release taskweft still succeeds on a machine without the toolchain (the entrypoint wiring can be assembled and inspected locally; the shippable binaries are produced in CI where zig is installed).